Synthetic aperture rfid handheld with tag location capability

ABSTRACT

A system for determining a bearing or location of a radio frequency identification (RFID) tag using a handheld RFID reader is described. In one embodiment, the reader is equipped with an accelerometer. A user moves the reader while the reader receives the tag&#39;s signal and determines the tag signal&#39;s phase at multiple locations. The locations of the reader antenna can be reconstructed using the accelerometer data. By using the phase determined at multiple locations in conjunction with the location of the reader antenna, the reader can determine the bearing of the tag. For an RFID reader not equipped with an accelerometer, the sign and ratio of the rate of change in the phase of a tag&#39;s signal to the distance traveled by the reader antenna can be used to determine the location of the tag relative to the reader.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of application Ser. No. 13/826,758 filed Mar. 14,2013, which claims the benefit of U.S. Provisional Application No.61/614,343 filed Mar. 22, 2012, and is hereby incorporated by referencein its entirety. This application is related to U.S. application Ser.No. 12/495,732 entitled, “METHOD AND SYSTEM TO DETERMINE THE POSITION,ORIENTATION, SIZE, AND MOVEMENT OF RFID TAGGED OBJECTS”, filed Jun. 30,2009, which granted as U.S. Pat. No. 8,248,210 on Aug. 21, 2012, and ishereby incorporated by reference in its entirety.

BACKGROUND

Handheld RFID readers are typically used to scan for any RFID tags thatare within range of the reader. Several hundred tags may respond, but aconventional RFID reader cannot identify the location of each individualtag. It would be useful for a reader to have the ability to locate thetags that respond to an RFID query.

However, conventional methods used to identify the location of a taginvolve either the use of highly directional steerable transmittingantenna arrays or highly directional steerable receiving antenna arrays.These antenna arrays are expensive and large, typically several feetacross. Another proposed method involves using a single motorizedantenna moving on a rail to create an equivalent synthetic aperture.However, none of these approaches are suitable for use with a compacthandheld RFID reader.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of a handheld RFID reader system are illustrated in thefigures. The examples and figures are illustrative rather than limiting.

FIG. 1 shows an example of a user's example hand motion while holding anRFID reader that receives tag responses during the hand motion todetermine the bearing of an RFID tag.

FIG. 2 is a flow diagram illustrating an example process of determininga bearing of an RFID tag.

FIG. 3 depicts a block diagram of an example RFID system for determiningthe bearing of an RFID tag.

FIG. 4A shows an example of a user's example hand motion while holdingan RFID reader not equipped with an accelerometer that can determine thelocation of tags relative to the reader.

FIG. 4B shows the locations of tags relative to an RFID reader andcalculated phases of the tags as a function of time or distancetraveled.

FIG. 5 is a flow diagram illustrating an example process of determininga location of an RFID tag relative to an RFID reader.

FIG. 6A shows an experimental layout for measuring locations of RFIDtags relative to an RFID reader. FIG. 6B shows a graph of calculated tagsignal phases as a function of distance traveled by the RFID readerbased on experimental data.

FIG. 7A shows a diagram of example sectors where an RFID tag may bedetermined to be located relative to the RFID reader.

FIG. 7B depicts a diagram of an example RFID radar display showing whereRFID tags may be located relative to the RFID reader.

FIG. 8 depicts a block diagram of an example RFID system having twoantennas.

DETAILED DESCRIPTION

A system is described for determining a bearing or location of an RFIDtag using an RFID reader equipped with an accelerometer. A user movesthe reader while the reader receives the tag's signal and determines thetag signal's phase at multiple locations. The locations of the readerantenna can be reconstructed using data from the accelerometer. By usingthe phase determined at multiple locations in conjunction with thelocation of the reader antenna, the reader can determine the bearing ofthe tag.

For an RFID reader not equipped with an accelerometer, the ratio andsign of the rate of change in the phase of a tag's signal to thedistance traveled by the reader antenna or the elapsed travel time canbe used to determine the location of the tag relative to the reader.

Various aspects and examples of the invention will now be described. Thefollowing description provides specific details for a thoroughunderstanding and enabling description of these examples. One skilled inthe art will understand, however, that the invention may be practicedwithout many of these details. Additionally, some well-known structuresor functions may not be shown or described in detail, so as to avoidunnecessarily obscuring the relevant description.

The terminology used in the description presented below is intended tobe interpreted in its broadest reasonable manner, even though it isbeing used in conjunction with a detailed description of certainspecific examples of the technology. Certain terms may even beemphasized below; however, any terminology intended to be interpreted inany restricted manner will be overtly and specifically defined as suchin this Detailed Description section.

Existing techniques use wireless communications for locating a tag, butthey are not suitable for use with a handheld RFID reader that operatesnear 900 MHz in the ultrahigh frequency (UHF) band. The size of a UHFantenna is generally on the order of the wavelength of the frequency.For a frequency of 900 MHz, the wavelength is approximately 30 cm.Consequently, for a highly directional high gain antenna, the size ofthe antenna attached to the RFID reader would be large and unwieldy.

Another technique for locating a transmitter uses multiple antennasarranged in a phased array. When a tag responds to a query, the phase ofthe tag signal received at each antenna is compared to determine thedirection of the tag. However, the phased array is also large andunwieldy.

The techniques presented below permit a handheld RFID reader with aninternal antenna that is not highly directional to determine, in onecase, an exact direction of the tag, and in another case, a location ofthe tag relative to the reader.

RFID Reader Equipped with Accelerometer

FIG. 1 shows an example of a user's hand motion while holding an RFIDreader that receives tag responses during the hand motion to determinethe bearing of an RFID tag. The locations of the reader antenna at themoment each of the tag responses are taken by the reader can becalculated using the accelerometer measurements to determine the bearingof an RFID tag.

The user 105 holds a handheld RFID reader 110 in his hand, and thereader 110 sends out an RFID query. When an RFID tag 120 responds to theRFID query, the RFID reader 110 receives the response and calculates thephase of the tag's response. To simulate receiving the tag response withmultiple antennas, the RFID reader takes readings of the tag responseswhile the RFID reader (and internal antenna) is moved to differentlocations by the user. While RFID readers typically average the responsefrom a tag to obtain a single phase value for the tag response, multiplephase data points can be obtained by a reader for each tag response.Additionally, the RFID reader can transmit more than one RFID query toelicit multiple responses from the tag while the reader is in motion.

The user can move the reader, for example, by swinging the reader in hishand. FIG. 1 shows an example trajectory 115 of the user's hand swingingin an arc from left to right in front of the user's body. However,movement of the reader in any direction will work, even in a zig-zagpattern, as long as the span of movement of the reader antenna issufficiently wide. Because the span of movement of the antennarepresents the equivalent synthetic aperture of a synthetic apertureantenna, the span of movement should be at least the wavelength of thetag signals. For UHF frequencies, the wavelength is approximately onefoot. Additionally or alternatively, the user can travel, for example ina forward direction, while holding the reader.

The reader 110 is equipped with an accelerometer, an inertialmeasurement unit or other system to measure reader movement that canmeasure acceleration in three dimensions. The accelerometer measures thethree components of acceleration, x, y, and z. By integrating theacceleration components over time, the velocity components are obtained,and by integrating the velocity components over time, the coordinatecomponents are obtained. The coordinate components are used to determinethe exact trajectory of the reader.

Measuring the phase of a tag's signal that is received at differentpoints along the reader's trajectory is the functional equivalent ofmeasuring a tag's signal using a phased antenna array. A phased antennaarray is made up of several antennas, and the phase of a signal receivedat each antenna is compared to determine the direction the tag signal iscoming from. With the phased array technique, it is critical to know theexact position of each antenna. The accelerometer data allows the exactposition of the reader to be determined for each tag signal measurementthat is taken.

An example reader equipped with an accelerometer can provideinstructions on the screen for how the user should move his hand, e.g.in a swinging motion from left to right or right to left, and/or hisbody, e.g. take three steps forward. The reader can also provideinstructions on when the user's movements should begin so that the tagmeasurements can be synchronized with the user's movements. Further, thereader can display on the screen the progress that the reader is makingin taking successful measurements and calculating the tag direction, forexample, in terms of probability. Once a certain level of confidence isreached, e.g. 90%, the user can be alerted that movement of the readercan be stopped.

Instead of or in addition to an accelerometer, the system may employ agyroscope. Angular acceleration readings from the accelerometer(s)and/or gyroscope can provide information about the relative motion ofthe reader, that is, whether the reader is moving from left to right, orfrom right to left. With this information, the reader can operate in amode where its display can further show arrows pointing to either theleft or the right side of the reader that indicate to the user in whichdirection the tag is located relative to the reader. If the reader ismoving from left to right and the phase is increasing, the tag will beto the right of the reader, and vice versa. Moreover, if there is notmuch angular acceleration, the reader can deduce that the user is movingthe reader toward or away from the tag, rather than sweeping the readerfrom side to side across a trajectory to the tag, and provide anindication on the display with arrows pointing to the top or bottom ofthe reader.

Changes in hand motion, e.g. change in speed, tilt, or an unsteadytrembling hand, have only a small effect on the accuracy of the tagsignal data used for making phase calculations. Consider an example casewhere the reader can read 1000 tags per second; a user's hand motion hasa speed of one meter per second during scanning; hand position variationalong the scan trajectory is ±1 inch; hand tilt variation along the scantrajectory is ±10 degrees; typical distance to the tag is 10 feet whichcorresponds to the maximum read range of a typical handheld RFID reader;the reader antenna is directional with a measurement of 6 dBi; the tagantenna is omnidirectional having a measurement of 2 dBi; and thefrequency is 915 MHz.

In free space, the received tag phase varies linearly with the distanced to the tag:

φ=−4πλd  (1)

When a tag is moved away from or towards the reader in free space, thetag signal phase changes by 360° (i.e., wraps around) for every halfwavelength of radial tag movement. With the above method, the phasedifference of the received tag signal is measured at two differentreceiving points along the user's hand trajectory. By using theaccelerometer data, the path difference, a, can be calculated, and thetwo-dimensional tag bearing can be approximately calculated as:

θ≈arc sin [−λ2π(φ2−φ1)a],  (2)

where a is the spacing between the two measurement points (which may becomputed based on output from the accelerometer).

Based on this free space theory and the assumed system parameters givenabove, the effects of unsteady hand motion on the accuracy ofdetermining the bearing of a tag can be determined and is seen to have asmall effect.

A finite tag identification rate of 1000 tags/sec over a hand sweep of 1meter corresponds to one measurement per millimeter of movement,resulting in a very small calculated tag bearing error, less than 1°.

Due to a variation in hand tilt of ±10 degrees, the calculated tagbearing error is also very small, less than 1°. The error is smallbecause the phase of the received signal depends on the antennaposition. The antenna radiation pattern can have a lot of gain variationwith angle, but the phase remains nearly constant with angle and mayvary only by 30° over a 90° tilt.

Due to hand position variation of ±1 inch, the calculated tag bearingerror is small, less than 5°, assuming that the tag signal measurementsare made along the full 1 meter hand trajectory, and the end points areused for the calculation.

In a multipath environment, there may be additional errors due toreflections from different objects, and the errors can be analyzed usinga RFID multipath propagation simulator. However, the majority of usecases for which the RFID reader is used are short range, directline-of-sight RFID applications.

FIG. 2 is a flow diagram illustrating an example process of determiningthe bearing of an RFID tag using an RFID reader equipped with athree-dimensional accelerometer. At block 205, the RFID reader transmitsan RFID query that is received by RFID tags in the vicinity of thereader.

Upon receiving the RFID query, one or more RFID tags transmit a responseto the query. At block 210, the accelerometer in the RFID readermeasures the acceleration in the x,y,z directions while at the sametime, at block 215 the RFID reader receives the responses of the tagsfor calculating the phases of the received tag signals.

In addition to needing the phase information for a tag at at least twopositions of the reader antenna, typically, more than one tag signaldata point is taken to ensure that the quality of the signal data isgood. Factors that can degrade the quality of the data include taking atag signal reading when the location of the reader antenna is at amultipath null and interference at the interrogation frequency, andconsequently, response frequency. Poor quality data, for example, datathat does not have a minimum signal strength, can be filtered out fromthe calculation of the tag's phase.

At decision block 220, the system determines whether to take more tagsignal readings. The system can base this decision on whether a minimumlevel of confidence has been reached. If the system has sufficient datato determine the bearing of the tag (block 220—No), at block 225 thesystem calculates the phase of the received tag signals. Then at block230, the system determines the trajectory of the reader from theaccelerometer readings and the position of the reader at the time thatthe tag signal phase measurements were taken. Then at block 235, thesystem determines the bearing of the tags using the tag phasecalculations and the location of the reader and provides the bearinginformation for each of the tags to the user at block 237.

If the system needs to take more measurements to determine the bearingof the tag (block 220—Yes), at decision block 240 the system determineswhether to change the interrogation frequency of the RFID query. Thesystem may decide to change the interrogation frequency for a fewreasons. Due to interference from certain frequencies in theenvironment, it may be advantageous to take tag readings at differentfrequencies to ensure that the data is of sufficiently high quality.Furthermore, in the United States, RFID systems typically operate in theunlicensed 915 MHz ISM (industrial, scientific, medical) band (902-928MHz). The Federal Communications Commission (FCC) mandates thattransmitters having a minimum output power use frequency hopping with alimit set on the maximum dwell time at any one frequency in the band.Thus, to comply with FCC regulations, the system may need to change theinterrogation frequency periodically.

If the system decides to maintain the same interrogation frequency(block 240—No), the process returns to block 205 to further query thetags. If the system decides to change the interrogation frequency (block240—Yes), at block 245, the reader selects a different interrogationfrequency. The process returns to block 205 to query more tags.

In one example, the RFID reader can read approximately 1000 tags persecond. If the user moves his hand a distance of three feet over a timespan of one second, and the reader changes frequencies every 0.3seconds, the reader can obtain approximately 300 data points for each ofthree frequencies over the course of the one second time span.

In another example, the reader can select frequencies located at theends of the frequency band of operation to ensure that any interferenceat one frequency is as far away in the frequency band as possible fromthe next selected frequency. Then the reader can hop back and forthbetween those frequencies to populate two data sets. Because there willbe multi-path reflections that cause nulls specific to a particularfrequency at particular locations, the calculated phases for each of thedata sets for each frequency can be averaged, or the data from the bestfrequency channel can be used.

FIG. 3 shows a block diagram 300 of an RFID reader used to read RFIDtags and determine the bearing of the tags. An RFID reader may includeone or more processors 305, memory units 320, power supplies 325,input/output devices 310, trajectory tracker 315, and RFID radio 330with antenna 335.

The accelerometer trajectory tracker 315 provides acceleration data forthe reader in three orthogonal directions, for example, a singlethree-dimensional accelerometer or three one-dimensional accelerometers.The trajectory tracker 315 can include any combination and number of:accelerometers, gyroscopes, GPS or similar geo-positioning receiverswhere a sufficient positioning signal can be received, and wireless LANor WAN receivers and ability to determine position/movement using knowntriangulation techniques.

A processor 305 may be used to run RFID reader applications. Theprocessor 305 can calculate the phase of a received tag signal andreconstruct the trajectory or relative position of the reader from theaccelerometer data and/or data from other devices that are able todetect movement and/or position of the reader to determine the bearingof the tag.

Memory 320 may include but is not limited to, RAM, ROM, and anycombination of volatile and non-volatile memory. A power supply 325 mayinclude, but is not limited to, a battery. An input/output device 310may include, but is not limited to, triggers to start and stop the RFIDreader or to initiate other RFID reader functions, visual displays,speakers, and communication devices that operate through wired orwireless communications. An RFID radio 330 includes standard componentsfor communication with RFID tags including an internal antenna 335.

RFID Reader Without Accelerometer

Although the use of accelerometer readings permits the determination ofthe bearing of a tag from the RFID reader fairly accurately, calculatingthe relative positions of the moving reader from the accelerometer datais complex.

It is possible to determine the relative tag location (front, back, orsides) with respect to an RFID reader without the use of anaccelerometer. A change in position of the reader antenna due to motionof the reader will result in a corresponding change in the phase of thereceived tag signal. By analyzing the change of phase of the tag signalas a function of antenna movement distance or, equivalently, elapsedmovement time, it is possible to determine the relative location of thetag. This method is the inverse of the time division-phase difference ofarrival (TD-PDOA) technique where instead of a having fixed readerantennas and moving tags, the tags are fixed and the reader antennamoves. The TD-PDOA technique is described in U.S. application Ser. No.12/495,732, entitled, “METHOD AND SYSTEM TO DETERMINE THE POSITION,ORIENTATION, SIZE, AND MOVEMENT OF RFID TAGGED OBJECTS”, filed Jun. 30,2009.

FIG. 4A shows an example of a user's hand motion while holding an RFIDreader, not equipped with an accelerometer, that can determine thelocation of tags relative to the reader. The user 450 holds a handheldRFID reader 455 in his hand, and the reader 455 sends out an RFID query.When an RFID tag 465 responds to the RFID query, the RFID reader 455receives the response and calculates the phase of the tag's response. Tosimulate receiving the tag response with multiple antennas, the RFIDreader 455 takes readings of the tag responses while the RFID reader(and internal antenna) is moved in a forward direction (or in anydirection, provided the reader is aware of the relative direction ofmovement).

Because the reader does not have an accelerometer, the exact distancethat the reader travels is not known. However, the exact distancetraveled is not necessary because the calculated phase can be plotted asa function of the time that the reader is moved in a particulardirection to determine the phase slope and a corresponding location ofthe tag relative to the reader. If the user moves the reader with aconstant velocity motion, the plot of the phase as a function of timewill be a smooth linear curve. If the movement of the reader is jerkysuch that the motion accelerates and/or decelerates, the curve will notbe as smooth. However, the general trend where the phase of the tagincreases or decreases with time still applies.

FIG. 4B shows an RFID reader 405 that has a low gain internal antennawith an omnidirectional pattern 410. There are four RFID tags in thevicinity of the reader 405. Tag 1 is located directly in front of thereader 405; tag 2 is located ahead and to the right of the reader 405;tag 3 is located directly behind the reader 405; and tag 4 is locatedbehind and to the left of the reader 405. The phase of each tag signalreceived at the reader 405 is dependent on the round-trip distancebetween the reader 405 and each of the respective tags. The round-tripdistance between each of tag 1, tag 2, tag 3, and tag 4 and the readeris represented by the respective lines 401, 402, 403, and 404.

The arrow 420 indicates a direction for an example motion of the reader405 made by the user. The reader motion can be performed by the usermoving his hand while holding the reader in the indicated direction, orby the user traveling in the indicated direction, or a combination ofboth. As the reader 405 moves in the indicated direction, the round-tripdistances 401, 402, 403, and 404 will change. Equation (1) above showsthe relationship between the tag phase and the distance between the tagand the reader. The graph on the right of FIG. 4 shows the phase of thereceived tag signal for each of the tags as a function of the distancetraveled or elapsed travel time by the reader 405 and its internalantenna.

The example motion 420 is linear and directly towards tag 1 and directlyaway from tag 3. Thus, the change of phase of the received signal forthese tags is largest as the reader is moved forward and,correspondingly, has the largest slopes in the graph. Tag 1 which is infront of the reader has a positive slope, and tag 3 which is behind thereader has a negative slope. When a tag is to the side of the reader,the slope of the phase as a function of distance traveled or elapsedtravel time will be intermediate to the slopes of the tags immediatelyahead of and behind the reader. For example, tag 2 is in front of thereader so it has a positive slope, but the slope is smaller than that oftag 1. Similarly, tag 4 is behind the reader so it has a negative slopewith a smaller magnitude than that of tag 3. Thus, the slope of thecalculated phase as a function of distance traveled by the reader orelapsed travel time can be used to determine the relative location ofthe tag with respect to the reader.

In one example, if the reader detects responses from, for example, 20RFID tags, based upon the change of phase with movement of the reader,the relative positions of the tags can be identified as one of theexample sectors shown in FIG. 7. A tag located in sector 1 would have alarge positive change in phase over an elapsed period of time (phaseslope), and a tag located in sector 3 would have a large negative phaseslope. A tag located in either sector 2A or sector 2B would have a small(or zero) positive phase slope, and a tag located in either sector 4A orsector 4B would have a small (or zero) negative phase slope.

In one configuration, the reader without an accelerometer can use anelectronic compass to determine when the reader is in motion. The dataobtained from the electronic compass can be used in conjunction withequation (1) to more accurately determine the phase slope, and thus, therelative location of a tag.

FIG. 6A shows an experimental layout where the RFID reader 605 has aninternal antenna with a substantially omnidirectional gain pattern. TagA is located directly in front of the reader. Tag D is located directlybehind the reader. Tag B is located ahead of the reader to the left, andtag C is located ahead of the reader to the right. As the reader 605 ismoved toward tag A, the reader receives signals from each of the tagsand calculates the phase.

FIG. 6B shows the calculated phase in degrees for each of the tags inthe experimental layout of FIG. 6A as a function of distance that thereader is moved. Tag A, the tag located in front of the direction inwhich the reader was moved, has the strongest positive slope, while TagD, the tag located in the opposite direction in which the reader wasmoved, has the strongest negative slope. Because tags B and C areintermediate in position between Tags A and D, they have slopes betweenthose of tags A and D.

FIG. 5 is a flow diagram illustrating an example process of determininga location of an RFID tag relative to an RFID reader. At block 502, thereader prompts the user to move the reader (and thus antenna) in aparticular direction. The direction can be selected by either the readeror the user, as long as the reader knows in which direction it will bemoved so that the relative locations where the tags are located can beaccurately assigned.

At block 505, the RFID reader transmits an RFID query that is receivedby RFID tags in the vicinity of the reader. Upon receiving the RFIDquery, one or more RFID tags transmit a response to the query. At block510, the RFID reader receives the responses of the tags.

Then at decision block 515, the reader determines whether to take moretag signal readings. If the system does not have sufficient data todetermine the relative location of the tag (block 515—Yes), at decisionblock 540, similar to decision block 240 in process 200 described above,the system decides whether to change the interrogation frequency of theRFID query. If the system decides to maintain the same interrogationfrequency (block 540—No), the process returns to block 505 to furtherquery the tags. If the system decides to change the interrogationfrequency (block 540—Yes), at block 545, the reader selects a differentinterrogation frequency. The process returns to block 505 to query moretags.

If the system has sufficient data to determine the relative location ofthe tag (block 515—No), at block 520 the system calculates the phase ofthe received tag signals. Then at block 525, the system determines thephase slope for each tag.

At block 530 the system provides the tag location information to theuser based on the calculated phase slopes. One way to provide the taglocation information is to list the tags and provide a correspondingarea for the relative location of the tag, such as the example sectorsshown in FIG. 7A. FIG. 7B depicts a diagram of an example RFID radardisplay showing where RFID tags of interest may be located relative tothe RFID reader.

Additionally or alternatively, the system can provide audio informationto the user about the proximity of the reader to an RFID tag. Forexample, the reader can provide an audio output similar to a Geigercounter where an audio output component of the reader emits a sound,such as a click or beep, when a tag has been read and identified.Subsequently, the rapidity with which the reader repeats the sound canbe used to convey whether the phase of the identified tag is increasingor decreasing. For example, the sound can be emitted more rapidly inresponse to a measured increase in phase, while the sound can be emittedless rapidly when the measured phase decreases. In some instances whenthe reader passes the identified tag, there will be a transition from anincreasing phase to a decreasing phase, or vice versa, and the rapidityof the emission can be set to be at a maximum when a transition isdetected.

Another way of providing audio information to the user about theproximity of the reader to the tag is to emit a low frequency tone whenthe tag of interest has been read and identified. Then, the phaseincreases when the user moves the reader and a phase wrap occurs, andthe frequency of the tone is increased. And when the phase decreases anda phase wrap occurs, the frequency of the tone is decreased. With bothof these audio techniques, the user can tell merely by listening to theaudio output of the reader whether the reader has passed the tag.

The block diagram of FIG. 3 showing the RFID reader equipped withaccelerometer can be adapted to apply to the RFID reader withoutaccelerometer by removing the accelerometer block 315. The other blocksfunction in the same manner as described above, except for theinput/output 310 and the processor 305. As described above, theinput/output 310 can also include an audio output component, such as aspeaker for providing audio information to the user.

The processor 305 can use a clock to time the motion of the reader todetermine a phase slope for each tag. The processor 305 can alsoidentify the locations of the tags relative to the reader from the phaseslopes. Further, the processor 305 can control the rapidity or period ofrepetition of the emission of a sound from the audio output componentand the frequency of the emitted tone of the sound.

RFID Reader With Dual Antennas

FIG. 8 depicts a block diagram of an example RFID reader system fordetermining the bearing of an RFID tag, where the reader is equippedwith an accelerometer and two internal antennas. The system componentsare the same ones as described in FIG. 3. Additionally, the (front)antenna 335 is located near the front of the RFID reader, while a rearantenna 336 is located near the rear of the RFID reader. For a typicalRFID reader, the front antenna 335 and the rear antenna 336 areseparated by nearly the length of the unit, approximately six inches. Byusing both antennas 335, 336 for receiving tag signals, the user doesnot need to move the reader as far to obtain the range of positions forthe antennas.

Similar to the adaptation of FIG. 3 for an RFID reader without anaccelerometer that can determine a location of a tag relative to thereader, FIG. 8 can also be modified by removing the accelerometer block315 to apply to the RFID reader without an accelerometer and similarlymodifying the function of the processor 305.

RFID Reader With Motorized Antenna

In one configuration, the RFID reader's antenna can be motorized to movethe antenna, for example, to extend outside the reader, or even totravel within the body of the reader. By motorizing the movement of theantenna, the user may not have to move the reader at all, or may need tomove the reader a smaller distance than without the use of the motorizedantenna. The motorized antenna can be used with both the RFID readerequipped with an accelerometer and the RFID reader without anaccelerometer.

CONCLUSION

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense (i.e., to say, in thesense of “including, but not limited to”), as opposed to an exclusive orexhaustive sense. As used herein, the terms “connected,” “coupled,” orany variant thereof means any connection or coupling, either direct orindirect, between two or more elements. Such a coupling or connectionbetween the elements can be physical, logical, or a combination thereof.Additionally, the words “herein,” “above,” “below,” and words of similarimport, when used in this application, refer to this application as awhole and not to any particular portions of this application. Where thecontext permits, words in the above Detailed Description using thesingular or plural number may also include the plural or singular numberrespectively. The word “or,” in reference to a list of two or moreitems, covers all of the following interpretations of the word: any ofthe items in the list, all of the items in the list, and any combinationof the items in the list.

The above Detailed Description of examples of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific examples for the invention are describedabove for illustrative purposes, various equivalent modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize. While processes or blocks are presented ina given order in this application, alternative implementations mayperform routines having steps performed in a different order, or employsystems having blocks in a different order. Some processes or blocks maybe deleted, moved, added, subdivided, combined, and/or modified toprovide alternative or subcombinations. Also, while processes or blocksare at times shown as being performed in series, these processes orblocks may instead be performed or implemented in parallel, or may beperformed at different times. Further any specific numbers noted hereinare only examples. It is understood that alternative implementations mayemploy differing values or ranges.

The various illustrations and teachings provided herein can also beapplied to systems other than the system described above. The elementsand acts of the various examples described above can be combined toprovide further implementations of the invention.

Any patents and applications and other references noted above, includingany that may be listed in accompanying filing papers, are incorporatedherein by reference. Aspects of the invention can be modified, ifnecessary, to employ the systems, functions, and concepts included insuch references to provide further implementations of the invention.

These and other changes can be made to the invention in light of theabove Detailed Description. While the above description describescertain examples of the invention, and describes the best modecontemplated, no matter how detailed the above appears in text, theinvention can be practiced in many ways. Details of the system may varyconsiderably in its specific implementation, while still beingencompassed by the invention disclosed herein. As noted above,particular terminology used when describing certain features or aspectsof the invention should not be taken to imply that the terminology isbeing redefined herein to be restricted to any specific characteristics,features, or aspects of the invention with which that terminology isassociated. In general, the terms used in the following claims shouldnot be construed to limit the invention to the specific examplesdisclosed in the specification, unless the above Detailed Descriptionsection explicitly defines such terms. Accordingly, the actual scope ofthe invention encompasses not only the disclosed examples, but also allequivalent ways of practicing or implementing the invention under theclaims.

While certain aspects of the invention are presented below in certainclaim forms, the applicant contemplates the various aspects of theinvention in any number of claim forms. For example, while only oneaspect of the invention is recited as a means-plus-function claim under35 U.S.C. §112, sixth paragraph, other aspects may likewise be embodiedas a means-plus-function claim, or in other forms, such as beingembodied in a computer-readable medium. (Any claims intended to betreated under 35 U.S.C. §112, 916 will begin with the words “meansfor.”) Accordingly, the applicant reserves the right to add additionalclaims after filing the application to pursue such additional claimforms for other aspects of the invention.

What is claimed is:
 1. A radio frequency identification (RFID) readercomprising: a location or movement determination component comprising asensor, the location or movement determination component configured tomeasure location or acceleration of the RFID reader; and an RFID radiocomprising a first antenna that transmits a first radio frequency (RF)query; wherein a first phase and a second phase for a tag signal sent bya tag is calculated in response to the first RF query and in response tothe RFID reader being moved, the first phase corresponding to the tagsignal received at a first position of the first antenna, and the secondphase corresponding to the tag signal received at a second position ofthe first antenna; wherein a trajectory or positions of the movingreader is calculated using data from the location or movementdetermination component; and wherein a bearing for the tag is determinedusing the calculated trajectory or positions and the calculated firstphase and second phase for the tag.
 2. The RFID reader of claim 1,wherein the at least one RFID radio comprises a second antenna, andwherein a third phase and a fourth phase corresponding to the tag signalreceived at the second antenna is calculated, and wherein determiningthe bearing for the tag uses the calculated third phase and fourth phasefor the tag.
 3. The RFID reader of claim 1, wherein the first antenna ismotorized.
 4. The RFID reader of claim 1, wherein the location ormovement determination component comprises a trajectory tracker.
 5. TheRFID reader of claim 4, further comprising an input/output componentthat is configured to display an indication of a direction of the tagrelative to the moving reader, wherein the location or movementdetermination component provides angular acceleration data for themoving reader, and wherein a location of the tag is determined from theangular acceleration data relative to the moving reader.
 6. The RFIDreader of claim 1, further comprising an input/output component that isconfigured to prompt the user when to move the reader to synchronizewith received tag signals.
 7. The RFID reader of claim 1, furthercomprising an input/output component that is configured to display atleast one of: progress in determining the bearing for the tag; and thedetermined bearing for the tag.
 8. The RFID reader of claim 1, whereinthe system determines whether to take more tag signal readings based onwhether a minimum level of confidence has been reached, and wherein theRFID radio transmits on a different frequency when the minimum level ofconfidence is not achieved within a predetermined time period.
 9. Aradio frequency identification (RFID) reader comprising: a RFID radiocomprising a first antenna that is configured to transmit a first radiofrequency (RF) query using the RFID radio; and wherein a processor isconfigured to calculate a first phase and a second phase for a tagsignal sent by a tag in response to the first RF query and in responseto moving the RFID reader in a first direction, wherein a first phasecorresponds to the tag signal received at a first position of the firstantenna, and wherein the second phase corresponds to the tag signalreceived at a second position of the first antenna, and wherein thefirst and second positions correspond to movement of the reader in thefirst direction, wherein a change of phase of the tag signal iscalculated as a function of time using the first phase and the secondphase, and wherein a location of the tag relative to the movementdirection of the reader is determined based on the calculated change ofphase as a function of time.
 10. The RFID reader of claim 9, wherein thefirst antenna is motorized.
 11. The RFID reader of claim 9, wherein theat least one RFID radio has a second antenna, and wherein the processoris further configured to calculate a third phase and a fourth phasecorresponding to the tag signal received at the second antenna, andwherein determining the bearing for the tag further uses the calculatedthird phase and fourth phase for the tag.
 12. The RFID reader of claim9, further comprising an input/output component that is furtherconfigured to display the determined location of the tag relative to themovement direction of the reader.
 13. The RFID reader of claim 9,wherein a positive change of phase as a function of time results whenthe direction of movement of the reader is toward the tag; and anegative change of phase as a function of time results when thedirection of movement of the reader is away from the tag.
 14. The RFIDreader of claim 9, wherein the RFID reader is identified when in motion,and wherein data from the RFID reader is used to determine the change ofphase of the tag as a function of time
 15. The RFID reader of claim 9,further comprising an input/output component that is configured to emita series of sounds, wherein characteristics of the series of sounds arecontrolled by the processor, and further wherein the processor isconfigured to change a period of repetition of the sounds in the seriesor a frequency of the series of sounds based upon the change of phase.16. A method of determining a location of a radio frequencyidentification (RFID) tag relative to an RFID reader, the methodcomprising: providing an RFID transceiver comprising an antennaconfigured to send a first RF query; calculating, by a processor, afirst phase and a second phase for a tag signal sent by a tag inresponse to the first RF query and in response to the reader moving in afirst direction, wherein a first phase corresponds to the tag signalreceived at a first location of the first antenna, wherein the secondphase corresponds to the tag signal received at a second location of thefirst antenna, and wherein the first and second locations correspond tomovement of the reader in the first direction; calculating a change ofphase of the tag signal as a function of distance traveled by the readerusing the first phase and the second phase; determining a position ofthe tag relative to the movement direction of the reader based on thecalculated change of phase as a function of time; outputting theposition of the tag relative.
 17. The method of claim 16, furthercomprising providing an indication of the change of phase
 18. The methodof claim 16, further comprising determining a confidence level of therelative location of the tag, and if a minimum level of confidence isnot satisfied, sending a second RF query using the RFID radio.
 19. Themethod of claim 18, wherein the second RF query uses a differentfrequency than the first RF query.